1. Field of the Invention
The present invention relates generally to blast cabinets, that is, enclosures in which workpieces can be blast-finished by a worker who manipulates a blast gun from outside the cabinet. In a more specific sense, the invention relates to a safety device for use on cabinets of this type, in the form of a basically pneumatic interlock system in which the supply of compressed air for operating a blast gun within the cabinet is shut down whenever the access door to the cabinet is not in a tightly closed position.
2. Description of the Prior Art
In blast cabinets of the type generally described above, an abrasive stream is discharged at high velocity from a blast gun against a workpiece supported within a blast chamber of the cabinet. Governmental regulations require, for the safety of individuals working in the vicinity of the blast cabinet, that the access door or doors to the blast chamber be tightly, sealably closed whenever the blast gun is in operation, since the abrasive-laden stream discharged from the gun or nozzle is potentially very hazardous to those working in the vicinity of the cabinet.
To this end, in the prior art various interlock systems between the door and the valve controlling the supply of air under pressure to the blast gun, have been devised. One commonly used method is to provide an electrically actuated main air valve having a solenoid coil connected in series with normally open limit switches provided on each door. Opening of either door opens one of the switches, preventing the supply of electric power to the main air supply valve. Such an arrangement has the disadvantage of requiring the mounting of switches near each cabinet door, with electrical wires mounted on the outside of the cabinet to connect each door switch in circuit with the solenoid valve used to control the main air supply. These components have traditionally been mounted on the cabinet exterior, because the abrasive environment within the cabinet makes it impractical to mount devices of this type within the blast chamber.
Another arrangement used in the prior art is of the pneumatic type, and has been typically used when the total air requirement is small. In these circumstances, normally closed two-way valves are mounted exteriorly of the cabinet adjacent the respective doors, controlling flow from the supply of air under pressure to the main control valve. The closing of the doors opens the two-way valves to allow the pressurized flow to the main valve.
In still other instances, the air flow requirements for the blast gun are too great for mounting the door-controlled valves directly in the main air supply line. Accordingly, in such an event, the main valve is typically actuated by air under pressure provided through a pilot air line. Again, valves mounted adjacent the doors are provided, but in this event are included in the pilot line rather than in the main air supply line. Opening of the doors closes the pilot-air-actuated valves, thus blocking the flow of pilot air under pressure to the main valve as an actuation signal therefor.
Prior art devices of the type described above have disadvantages in that the cost of mounting switches and valves individual to the doors of a blast cabinet should, as will be understood, be eliminated if at all possible. And, even when this cost is incurred for switches or valves of high quality individual to the doors of the blast cabinet, these components have the disadvantage in that they have moving parts, and must be rendered impervious to contamination by dust and abrasive particulates normally present in the ambient atmosphere of the work area. The failure to tightly seal limit switches and door-operated valves is likely to produce, over a period of time, malfunctions that require time and expense to correct, and that adds to down time for the complete blast finishing system in which the improperly functioning components are incorporated.
Further, in the prior art arrangements discussed above, a door switch or door operated valve, should it go out of adjustment, fail, or malfunction, will immediately shut down the entire unit. In such an instance the cause of the shutdown is not immediately obvious to the operator, and considerable time may be lost before the cause is determined.
The present invention aims to overcome the above noted deficiencies characteristic of the prior art devices conceived for the same purpose as the present invention. To this end, the present invention avoids the requirement of limit switches or valves individual to and mounted adjacent the access doors of the blast cabinet, along with the special provisions for sealing them to prevent malfunction. The invention further is designed to eliminate, along with the limit switches and door operated valves, exterior wires or piping on the doors and door frame, all of which have heretofore not only detracted from the appearance of the equipment, but also have had the disadvantage of defining projections that can be accidentally struck, broken off, or pulled away in a manner to cause a safety hazard to adjacent workers, and/or damage to the equipment itself.